The present invention relates generally to wireless communication systems and in particular to a technique for dynamic allocation of remotely deployed transceiving equipment.
Wireless communication networks, such as cellular mobile telephone and Personal Communications Services (PCS), continue to enjoy wide spread growth and popularity. There is often times a need in such systems to provide increasingly greater call handling capacity, as well as to accommodate higher peak usage. Emerging PCS networks, presently still in the stages of being implemented, demand additional design considerations such as low build out cost as they must compete with entrenched cellular networks.
Several approaches have been adopted for deploying such networks. One approach is to increase the coverage area afforded by a given system by increasing the antenna tower height and transmit power level beyond conventionally accepted norms. However, such solutions often increase the number of xe2x80x9cblindxe2x80x9d spots in areas that include a number of tall buildings, hills, or other natural obstructions to radio propagation.
Alternatively, a relatively large number of base stations may be deployed with smaller radio coverage xe2x80x9cfootprintsxe2x80x9d. While this avoids blind spots, it greatly increases the total capital cost for base station transceiving equipment which may be $200,000 or more per cell site.
Rather than deploy base station equipment in each relatively small cell (which would be relatively cost prohibitive), broadband distribution cable networks can be used to connect the antennas to centrally located base station equipment. For example, a suggestion has been made in U.S. Pat. No. 5,381,459 to use cable television networks to distribute wireless communication signals between base transceiver system (BTS) equipment and remote transceiver sites located at each cell. This approach couples the transceiver signals over an existing coaxial cable television network using time or frequency division multiplexing in order to avoid interference with other signals being carried, such as cable television signals.
Recently, other types of broadband distribution networks have also been proposed. Such networks consist of optical fiber transmission media which can directly distribute signals between centrally located base transceiver system (BTS) equipment and remotely located transceiver equipment. See, for example, our co-pending United States"" patent application Ser. No. 09/256,244 entitled xe2x80x9cOptical Simulcast Network with Centralized Call Processing,xe2x80x9d filed Feb. 23, 1999.
There is also presently a demand by the customers of such cellular telephone systems for digital modulation techniques, such as code division multiple access (CDMA). In these CDMA systems, such as the IS-95B system being used widely in the United States a common frequency band is used to support communication between multiple mobile subscriber units and base stations. With this technique, signals occupying a common carrier frequency are discriminated at a receiving terminal (which may either be the base station or the mobile unit) based on the use of pseudo random noise (PN) codes. In particular, transmitting terminals use different PN codes or PN code phase offsets to produce signals that may be separately received. The mobile unit is then provided with a list of carrier signal codes and phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is also equipped with a searching function that allows it to track the strength of the carrier signals generated from a group of the neighboring base stations.
In this CDMA system, various methods exist for switching a mobile unit from one base station to another. These methods, known as xe2x80x9chandoff,xe2x80x9d are an essential feature of cellular telephone systems which must support the ability to continue a telephone conversation in progress as a mobile unit moves between cells. The handoff method specified in the most popular CDMA system standards is called a xe2x80x9csoft handoff.xe2x80x9d This method is considered xe2x80x9csoftxe2x80x9d in the sense that communication with the adjacent base station is established before communication is terminated with the original base station. While the mobile unit is communicating with both base stations, a single receive signal for the remote subscriber unit is created by combining the signals from each base station within the circuits located in the mobile unit. Similarly, the signals received from the mobile unit by both base stations are combined in a centralized system controller prior to being forwarded to complete the connection.
While soft handoff solves certain problems caused by the movement of mobile units between cells, other difficulties are encountered within such systems when they use broadband distribution networks to distribute signals between remotely located transceiver equipment and the centralized base station equipment. In such networks, it is desirable to utilize the sharing or xe2x80x9csimulcastxe2x80x9d of radio carriers in adjacent cells. This permits the most efficient use of radio transceiving equipment when the demand for use of the system is relatively low.
However, as traffic demand increases over short periods of time, such as when traffic patterns change during the course of a day, it becomes desirable to activate additional transceiving equipment in the cells. By enabling the xe2x80x9cblossomingxe2x80x9d of such radio coverage, the additionally activated transceiving equipment can handle the increased traffic load. Such equipment should be deployed in a way which avoids the need for the remote units to switch between carrier frequencies. In particular, it would be desirable to avoid having to interrupt a communication in progress to command a mobile unit to perform a xe2x80x9chardxe2x80x9d handoff to switch to a different carrier.
In other words, the system should operate in a simulcast mode such that adjacent cells or sectors may use the same carrier and code phase offsets when the traffic density is relatively light. It would then be desirable to disable the simulcast as new capacity is needed, and to do this in a way which does not require modification of standard remote subscriber units such which are already in use.
Briefly, the present invention is a technique for handling changes in demand over short periods of time in a is wireless communication system. An optical fiber or other available broadband distribution network is used to distribute signals between Centrally located base transceiver station (BTS) equipment and remotely located transceiver equipment referred to herein as xe2x80x9ccable microcell integratorsxe2x80x9d (CMI). The CMIs are deployed in a configuration such as one per cell (or cell sector) to provide radio frequency coverage in a pattern which approximates the eventual expected required deployment of base stations when the system is at full capacity.
With this scenario, a single radio carrier preferably carries the channelized radio frequency (RF) signals as a simulcast for a number of different CMIs. The same active traffic channels may therefore be broadcast to multiple CMIs and hence to multiple coverage areas during time periods of low demand. In this mode, multiple adjacent CMIs are configured to communicate with the mobile subscriber units using the same RF channel. A group of CMIs arranged in this manner are referred to as a xe2x80x9csimulcast cluster.xe2x80x9d Simulcast clusters may also be defined by assigning other signal characteristics in common. For example, in CDMA systems, simulcast clusters are defined by assigning a common carrier frequency, common pseudonoise (PN) code, and common PN code phase offset. In comparison to traditional networks wherein the full capacity of an RF channel is not fully utilized, the coverage area of an RF channel may therefore be extended via the simulcast to provide a significant improvement in network efficiency.
In order to accommodate changes in traffic demand, such as may occur during a rush hour, a second RF channel is activated within the RF coverage area of at least one CMI. This second RF channel is provided by deploying an auxiliary CMI or auxiliary transceiver within the original CMI. The power level of this second RF channel is brought up gradually so that the system may rely upon the soft handoff features built into the subscriber units. In this manner, as the subscriber units acquire sufficient receive power from the second RF channel, a number of the subscriber units are automatically switched over to the second RF channel due to their own internal soft handoff processing.
In effect, the remote subscriber units are fooled, or xe2x80x9cspoofedxe2x80x9d into thinking that they are moving into a new cell, e.g., that they are moving closer to a base station in an adjacent sector operating with the second channel when, in actuality, they may not be moving at all. As a result, a subset of the mobile units within the cell will be switched over to the second carrier frequency, in effect splitting the traffic demand in the cell among the two carriers.
As traffic demand drops, such as towards the end of the rush hour, the power level of the second channel is slowly decreased. This causes the remote subscriber units in that section to begin to hunt for a stronger carrier which will be, for example, the original channel. At some point, they will switch over to the original channel, and the system returns to its original state.
Now more particularly, base station sector call capacity is initially distributed across the RF coverage area spanned by the simulcast of CMIs which are connected to the sector of interest. As caller demand increases, it is desired to transfer active calls to an additional base station sector without dropping or otherwise corrupting the calls. Simply reassigning one or more of the CMIs in simulcast mode to an auxiliary base station sector will add capacity within the RF coverage area in question, but it will also result in the dropping of mobile calls within the coverage area of the CMIs being reassigned. To remedy this, an auxiliary base station sector is first activated within the area of one or more CMIs in the simulcast. The original base station signal is then removed from that same area of one or more CMIs. This may be accomplished through the use of additional CMIs or by providing the CMIs with multiple RF carrier capability. As an example, consider a CDMA simulcast network of N CMIs distributing base station call capacity of K mobile calls across the simulcast. When call demand exceeds K calls within the region, it is desired to add another K call capacity within the region using the same RF carrier frequency but a different PN offset thus maintaining soft hand-off call capacity within the region of N CMIs. Eventually, the distributed network will have M CMIs simulcasting the auxiliary base station sector while N-M CMIs will be simulcasting the original base station sector. Abruptly switching the M CMIs to the auxiliary sector will drop mobile calls within the RF coverage area of the M CMIs being reallocated. In the present invention, the auxiliary sector (additional RF carrier at the same frequency but with a different PN offset) is activated at or near the location of the M CMIs being reallocated. In the preferred embodiment, the additional carrier is gradually increased to minimize the rate at which mobile calls begin soft handoff operations with the initial base station sector and the auxiliary base station sector.
As the auxiliary base station sector is activated at or near the M CMIs being reallocated, the initial RF carrier from the original base station sector is deactivated. In the preferred embodiment, the deactivation gradually lowers the forward link RF transmitted power levels to minimize the rate at which mobile calls terminate their soft handoff operations. When the addition of additional call capacity is complete, there is single RF carrier frequency radiated at any one CMI site, but M of them operate with the new PN offset and N-M of them operate at the original PN offset. This technique is not limited to the addition to a single base station sector within a CMI simulcast cluster, but can continue with the addition of a third base station sector. In the preferred embodiment, each sector is brought into the network one at a time. This limits the xe2x80x9cspoofingxe2x80x9d generated soft handoffs to two way handoffs.
When traffic demand decreases, the reverse operation is performed to remove the auxiliary base station sector from the simulcast cluster. At or near the locations of the M CMIs that are transmitting the auxiliary base station sector, the original base station sector is activated, thus initiating soft hand-off operations for all mobile calls within the area of RF coverage of the M CMIs. Then the RF carrier associated with the auxiliary base station is de-activated leaving all N CMIs in a common simulcast group connected to the initial base station sector.
For air interfaces other than CDMA, the operations are the same except that soft hand-off operations are not realized during the blossom and wilt transitions. However, the co-existence of the original and auxiliary carrier is still needed to prevent call drops. The overlap time for other air interfaces is used for call set-up to allow hard hand-off operations, which otherwise could not be executed. During the overlap period, the mobile makes carrier power measurements, e.g., performing mobile assisted hand off (MAHO), and both the original and auxiliary base station scanning receivers measure reverse link received signal quality prior to executing the hand-off decision.